In recent years, a secondary battery, such as a lead storage battery or a nickel hydrogen battery, has been used as a large-size and large-capacity power source, a typical example thereof being a power source for an electric vehicle, a hybrid car, an electric motorbike or a forklift. Recently, developments have been actively made to adopt a lithium ion secondary battery, which is high in energy density. In the developments, increases in the size and the capacity thereof have been advanced while an improvement in the cycle life, the safety and others has been taken into consideration.
As a power source for the above-mentioned vehicles or articles, use is made of a battery pack in which a large number of batteries are connected to each other in series or in parallel since the pack gives a large driving power.
In order to interconnect batteries to each other so as to connect the batteries in series or in parallel, or take out electric energy from the batteries, bus bars are connected to external terminals of the batteries (see, for example, JP-A 2009-87542 (KOKAI), JP-A 2009-87720 (KOKAI), JP-A 2009-87722 (KOKAI) and JP-A 2009-87761 (KOKAI)). The method for connecting bus bars to external terminals is generally a method of making holes in the bus bars, and fastening the hole-made bus bars to bolts formed in the external terminals with screws. A contact resistance is generated between the external terminals and the bus bars fastened thereto with the screws. It is feared that when vibration, or a mechanical force such as impact force is applied to the sections fastened with the screws, the contact resistance is changed.